Presently printers, particularly computer controlled printers, are extensively used to create various types of documents. In some instances printers create the entire document. In other instances, printers merely fill in variable text on preprinted forms that include lines and fixed (e.g., nonvariable) text. For example, airline tickets are preprinted forms on which the traveler's name and flight data is printed. In order to avoid the need to insert blank sheets of paper or preprinted forms each time a document is to be created, paper and many forms are produced in continuous sheets. Each element of the sheets--separate form or piece of blank paper--is separated from the adjacent elements by perforations that allow printed documents to be separated from the continuous sheet by "tearing" the printed document from the continuous sheet after the printed document exits from the printer. Usually the sheets are fan folded at the perforations.
In many printers, continous sheets are moved through the printer by tractors. Tractors include drive belts or wheels with pins that engage holes located in tear strips positioned along the edge of the continuous sheets. Tractor feeds have a number of advantages. First, they prevent skewing of the continuous sheets as they are moved through the printer. Further, they maintain the alignment of multiple layer continuous sheets if edge holes are formed in all of the layers.
The tractors of most printers are located downstream of the print mechanism, i.e., downstream of the printer platen. These tractors pull the continuous sheets through the printer. In order to avoid having to continuously retread the tractors, printed documents are moved to a point downstream of the tractors before they are removed by tearing along the separating perforations. This usually results in the creation of an unprinted region between documents since the tractors are usually located several inches downstream of the platen. While such a loss is acceptable when long documents are printed on inexpensive blank paper or when preprinted information is positioned near the top of a document or when a large blank heading area near the top of a document is needed or desired, it is unacceptable when short documents on preprinted forms are printed, particularly when the forms are expensive multiple copy forms. This problem has been avoided by placing tractors upstream of the print mechanism and using them to push continuous sheets into the printer, which is friction fed by the platen and rollers past the print head. The use of upstream tractors avoids the skewing problem that occurs with printers that rely solely on friction feed platen/roller mechanisms. In order to avoid paper buckling between upstream tractors and the printer platen, the printer platens of printers with upstream tractors are sized and/or driven such that the platen peripheral speed is slightly greater than the tractor push speed.
While the use of upstream tractors allows printed documents to be separated from a continuous sheet at a location slightly beyond the print head, acceptable separation frequently requires the application of a holding force to the "next" element of a continuous sheet as the printed document is being separated. That is, a tear bar mechanism is needed to press the next element of a continuous sheet against the printer platen when a printed document is being separated. For various reasons, prior tear bar mechanisms have not proven to be entirely satisfactory. In some instances, they have been located an undesirable distance downstream of the print head. In some instances, the tear bar has worked satisfactorily with one weight of continous paper but not with other weights. Multiple copy or multiple layer continuous sheets have also created problems. The present invention is directed to providing a tear bar mechanism that avoids these and other problems.